In the field of dot matrix printers, it has been quite common to provide a print head which has included therein a plurality of print wire actuators or solenoids arranged or grouped in a manner to drive the respective print wires a precise distance from a rest or non-printing position to an impact or printing position. The print wires are generally secured to the solenoid plunger or armature which is caused to be moved such precise distance when the solenoid coil is energized and wherein the plunger normally operates against the action of a return spring.
It has also been quite common to provide an arrangement or grouping of such solenoids in a circular configuration to take advantage of reduced space available in the manner of locating the print wires in that area from the solenoid to the front tip of the print head near the record media. In this respect, the actuating ends of the print wires are spaced in accordance with the circular arrangement and the operating or working ends of the print wires are closely spaced in vertically aligned manner adjacent the record media. The availability of narrow or compact actuators permits a narrower print head to be used and thereby reduces the width of the printer because of the reduced clearance at the ends of the print line. The print head can also be made shorter because the narrow actuators can be placed in side-by-side manner closer to the record media for a given amount of wire curvature.
Further, it is also common to provide a plurality of single wire actuators or solenoids which may be equally spaced one from another along a line of printing and wherein the spacing between the solenoids is generally in the range of one half to one inch. These single wire actuators or solenoids are generally tubular or cylindrically shaped and include a shell which encloses a coil, an armature and a resilient member arranged in manner and form wherein the actuator is operable to cause the print wire to be axially moved a small precise distance in dot matrix printing.
The axial wire solenoids are typically built from cylindrical components for two major reasons. First, it is said that the force generated by the magnetic field is proportional to the change in permeance of the magnetic circuit with air gap length. Permeance is defined as the reciprocal of reluctance, and further defined that in a plane through any cross section of a tubular portion of a magnetic circuit bounded by lines of force and by two equipotential surfaces, is the ratio of the flux to the magnetic potential difference between the surfaces under consideration. A certain amount of the fringing flux exists at the edges of the air gap in a solenoid and due to the circuitous path of the flux the fringing flux contributes a small amount or proportion of the force generated in the solenoid. A circular cross-section at the pole face will obviously have the smallest perimeter and the least amount of fringing flux. Secondly, it should be noted that a cylindrical design employs both a cylindrical armature and a cylindrical coil closely coupled therewith, with the result that less copper per turn is utilized in the assembly of the solenoid, thereby increasing the efficiency of the unit.
While the cylindrical units can claim certain advantages as mentioned just above, it is seen that small and compact actuators of different configuration and construction are advantageous along a line of printing for effecting or enabling an increase in throughput or an improvement in print quality. Such small and compact actuators of rectangular design allow a reduction in distance between print wires of adjacent actuators and also reduce the length of the print head.
Representative documentation in the design of a matrix print actuator includes U.S. Pat. No. 3,755,700, issued to H. Buschmann et al. on Aug. 28, 1973, which discloses an electromagnetic drive having an armature guided in the pole shoes and attached to a print needle. The armature has a portion formed to receive a helical spring at one end and the other end engages a threaded pin.
U.S. Pat. No. 3,775,714, issued to H. Heuer on Nov. 27, 1973, includes a core portion and a plunger freely disposed within the sleeve-shaped inner core portion and operatively connected with indicating means.
U.S. Pat. No. 3,850,278, issued to H. P. Mihm et al. on Nov. 26, 1974, discloses a printing needle which is connected to the armature of an electromagnet by means of a thickened portion at one end of the needle and then fastened to the armature by peening, or the needle could be connected by flattening the end of the needle and then pivoting same to a hinge type of armature.
U.S. Pat. No. 4,016,965, issued to R. L. Wirth on Apr. 12, 1977, discloses a solenoid driver wherein the print wire is fastened to the plunger and the flanged end of the plunger provides a seat for the return spring and also a flux path from the air gap to the plunger.
U.S. Pat. No. 4,034,841, issued to S. Ohyama et al. on July 12, 1977, shows a print wire solenoid device including a disc type plunger restoring spring which is not secured at any point to the plunger nor to adjacent components, and the device is made with a conical formation of the plunger and stem members to reduce size and weight of the device.
U.S. Pat. No. 4,137,513, issued to J. W. Reece et al. on Jan. 30, 1979, discloses a matrix print wire solenoid having an armature or plunger core secured to the print wire and a return spring has one end seated over a portion of the armature and the other end seated in a recess of a plastic bushing.
U.S. Pat. No. 4,165,940, issued to C. T. Cacciola on Aug. 28, 1979, discloses a free flight, ballistic type head assembly with print wires and springs engageable with a wire end cap and an armature assembly.
U.S. Pat. No. 4,211,495, issued to R. E. Einem et al., discloses a print head made with a plastic lamination and one or two grooved plates which are initially flat and easy to mold in a manner for close solenoid spacing.
U.S. Pat. No. 4,211,496, issued to J. E. Naylor on July 8, 1980, discloses a printing solenoid with a compression spring biasing an armature at one end and engageable with a front guide piece.
And, U.S. Pat. No. 4,218,150, issued to R. L. Swaim on Aug. 19, 1980, discloses a moving coil mechanism in a matrix printer wherein the mechanisms are built in a nested arrangement to contribute to the compactness of the printer.